(1) Field of the Invention
The present invention relates to a tubular tissue scaffold, and, in particular, to a tubular tissue scaffold having aligned biopolymer fibrils, for use in tissue engineering applications; an apparatus and method of producing a tubular tissue scaffold having aligned biopolymer fibrils; and artificial tissue, and methods of use thereof, comprising living cells attached to a tubular tissue scaffold having aligned biopolymer fibrils.
(2) Description of the Related Art
The National Science Foundation defines tissue engineering as “the application of principles and methods of engineering and life sciences to obtain a fundamental understanding of structure-function relationships in novel and pathological mammalian tissues and the development of biological substitutes to restore, maintain, or improve [tissue] function.” See Shalak, R. and Fox, J. eds., Tissue Engineering, Proceedings of a Workshop held at Granlibakken, Lake Tahoe, Calif., Feb. 26-29, 1988, New York: Alan Liss (1988). In the last decade, over $3.5 billion dollars has been invested worldwide in tissue engineering research. More than 70 start-up companies or businesses having a combined annual expenditure of over $600 million dollars now participate in a significant engineering and scientific effort toward developing alternative sources of transplant materials through in vitro tissue engineering.
Several aspects of creating an engineered tissue make it a daunting task. One of the most difficult challenges is directing the behavior of specialized cells outside of the body to mimic the normal, endogenous phenotype those cells exhibit in vivo. Additionally, in order for an engineered tissue to be tolerated upon implantation, the material that provides the scaffolding for the cells must meet several important criteria. The material must be biocompatible, so as not to be toxic or injurious, and not cause immunological rejection. Also, the material must be biodegradable, by having the capability of being broken down into innocuous products in the body. Because cells respond biologically to the substrate on which they adhere, the materials that provide the growth surface for engineered tissues must promote cell growth. Further, the scaffolding material should be replaced by extracellular matrix components secreted by the grafted cells as the scaffold is broken down in the body. Additionally, the material should allow cells to grow and function as they would in vivo.
Initially, researchers adapted synthetic degradable polyesters that had been used in surgical materials since the early 1970s to construct scaffold materials for use in tissue engineering. These degradable polyesters include polyglycolide and polylactide, as well as the more recently developed polymer, polylactide coglycolide. However, those degradable polyesters tended to be inflexible, and their degradation in vivo has been associated with adverse tissue reactions. These shortcomings have led to the development of a host of new synthetic polymers, for example, polyhydroxybutyrate and copolymers of hydroxybutyrate with hydroxyvalerate. See Amass, W. et al., Polymer Int, 47:89-144 (1998).
In animals, collagens make up a majority of endogenous scaffolding materials. They are the most commonly occurring proteins in the human body and they play a central role in the formation of extracellular matrix. Collagens are triple-helical structural proteins. It is this triple-helical structure that gives collagens the strength and stability that are central to their physiological role in the structure and support of the tissues in the body. Although there are over twenty types of mammalian collagens, collagen types I, II, III, V, and XI make up the fibrous collagens. Type I collagen molecules polymerize into fibrils which closely associate in a parallel fashion to form fibers with enormous tensile strength, which are found in skin, tendon, bone and dentin. Type II is the major collagen found in cartilage, where the fibrils are randomly oriented to impart both stiffness and compressibility to the proteoglycan matrix. Type III collagen is found in skin, muscle, and vascular structures, frequently together with type I collagen.
Collagen has been used successfully in several tissue engineering applications. As a copolymer with glycoaminoglycans such as chondroitin 6-sulfate, collagen has been utilized as an artificial skin scaffold to induce regeneration in vivo for the treatment of burn injury since the early 1980s. See Burke, J. F., et al., Ann Surg, 194:413-28 (1981); Yannis, I. V., et al., Science, 215:174-6 (1982). Collagen has also been used to form anti-adhesion barriers for use on surgical wounds. See U.S. Pat. Nos. 5,201,745 and 6,391,939 to Tayot et al.; U.S. Pat. No. 6,451,032 to Ory et al. Zilla et al., in U.S. Pat. No. 6,554,857, describe the use of collagen, among other materials, as a component in a concentric multilayer ingrowth matrix that can have a tubular form.
However, some problems remain to be solved in the use of collagen as a scaffolding material for applications requiring structural and mechanical stability, such as for vascular prosthetics. This is at least partly due to an inability to isolate collagen possessing the physical properties required to maintain necessary mechanical integrity of a scaffold, as it is remodeled in vivo, for use in, for example, cardiovascular indications. Additionally, in order for a tubular construct to mimic endogenous components of the cardiovascular system, it must promote the proper growth, orientation, association, and function of specialized cell types.
Despite significant work in the field of tissue engineering and the numerous synthetic biomaterials that have been developed in the last decade, there is still a need for improved scaffolding materials for use in specialized applications. It would be useful, therefore, to provide a tissue scaffold in the form of a tube, comprising a biopolymer which promotes maintenance of an in vivo cell phenotype and, particularly, a tubular tissue scaffold that was non-toxic, biologically degradable in vivo, and causes little or no immune reaction in a host. It would also be useful to provide an apparatus and method for the production of such a tubular tissue scaffold. Also, despite the advances in biomaterials research, and the elucidation of the molecular biology of cell behavior and cell:matrix interactions, the gap between in vitro engineered tissue and biologically functional implantable organs remains significant. Therefore, it would be useful to provide artificial tissue that can act as a functional prosthetic. It would also be useful if the artificial tissue could be produced in the form of a tube, utilizing the tissue scaffolding described herein. This structural configuration would be particularly useful in cardiovascular applications.